Emission control devices, such as diesel particulate filters (DPF), may reduce the amount of particulate matter emissions (such as, soot) from a diesel engine by trapping the particles. Such devices may be regenerated during engine operation to decrease the amount of trapped particulate matter (for example, by burning) and maintain the collection capacity of the device. To meet stringent federal government emissions standards, regeneration operations and DPF functionality may be tightly controlled and regularly assessed.
One example approach for controlling diesel particulate filters is illustrated by Stewart et al. in U.S. Pat. No. 7,155,334. Therein, an engine controller initiates filter regeneration based on inputs received from sensors, such as particulate matter sensors and/or carbon dioxide sensors, positioned upstream and downstream of the filter. Specifically, the controller determines whether to regenerate the filter based on the composition of exhaust gas before and after passage through the filter.
However, the inventor herein has recognized issues with such an approach. As one example, the use of resistive sensing-based particulate matter (PM) sensors reduces the sensitivity of the emission control system. As such, commonly used PM sensors may be configured to detect the presence of PMs electrically, based on a change in resistance or capacitance across an electrical circuit. Such sensors may have a “dead-band” during which PMs may have to accumulate before the sensor is able to respond. This additional time required to detect PMs may delay the determination and initiation of filter regeneration. As such, the delay in filter regeneration may accelerate filter degradation. The additional time may also reduce the electrical sensor's ability to identify DPF degradation. Overall, this may lead to degraded exhaust emission levels.
As another example, the use of input from CO2 sensors that sense exhaust CO2 levels in determining filter regeneration may reduce the system's ability to accurately estimate the soot load on the filter due to an indirect correlation between filter soot levels and exhaust CO2 levels. Since the exhaust CO2 level is more representative of combustion conditions, a soot load may be inferred but not accurately determined.
Thus, in one example, some of the above issues may be addressed by a method of operating an engine exhaust system including a particulate filter comprising adjusting engine operation based on a CO2 signature of oxidized, post-filter exhaust particulate matters (PMs). The CO2 signature may include a CO2 level of oxidized PMs estimated by a CO2 sensor positioned downstream of the filter.
In one example, a diesel engine exhaust system may be configured with a filter substrate and a CO2 sensor positioned downstream of a DPF. During selected engine running conditions, an engine controller may heat the substrate and oxidize post-filter exhaust particulate matters (that is, exhaust soot) on the heated substrate using oxygen present in the exhaust gas. The CO2 generated from the oxidation of the soot may be estimated by the downstream CO2 sensor to determine a CO2 signature of the oxidized, post-filter exhaust particulate matters (PMs). The CO2 signature may at least include a CO2 level of the oxidized PMs. Since the generated CO2 is largely dependent on the quantity of exhaust soot oxidized on the heated substrate, a direct correlation may be made between the estimated exhaust CO2 level and an exhaust soot level. In other words, the CO2 sensor may be used as a PM sensor. The controller may then adjust engine operation and perform filter diagnostics based on the CO2 signature. During other engine running conditions, the CO2 sensor may be used to sense an exhaust CO2 level unrelated to post-filter exhaust PMs.
For example, during a first engine running condition, when the filter is storing and the substrate is not oxidizing (e.g., a substrate heater is turned off), the engine controller may adjust engine operations (such as, EGR operations) based on an exhaust CO2 level estimated by the CO2 sensor. During a second engine running condition, different from the first engine running condition, when the filter is storing and the substrate is oxidizing (e.g., a substrate heater is turned on), the engine controller may adjust engine operations (such as, initiation of filter regeneration) and perform filter diagnostics (such as, determine that a filter is leaking) based on the estimated CO2 level of oxidized, post-filter exhaust PMs. As such, during such conditions, low to substantially no exhaust PMs may be expected in the post-filter exhaust. Herein, by comparing the output of the CO2 sensor to CO2 levels expected based on the engine's operating conditions, exhaust PMs may be identified in the post-filter exhaust and may be used to infer filter degradation. For example, the CO2 sensor may sense post-filter exhaust CO2 levels in real-time to provide a real-time indication of the presence of PMs in the exhaust. In response to the estimated CO2 level (that is, sensor output) being greater than the expected CO2 level, an engine controller may indicate filter degradation due to the presence of PMs in the exhaust. In the event of filter degradation, the controller may further adjust engine operating conditions, and regeneration conditions, in response to the indication of degradation. In comparison, if the estimated level is not greater than the expected level, but is greater than a threshold, the controller may infer that the filter is not degraded, but that sufficient soot has accumulated on the upstream filter, and that a filter regeneration operation should be initiated. Accordingly, filter regeneration may be initiated to restore the filter's collection capacity.
It will be appreciated that while the depicted example illustrates application of the CO2 sensor in a diesel engine exhaust system, this is not meant to be limiting, and the same CO2 sensor may be similarly applied in alternate engine exhaust systems, such as to diagnose a gasoline particulate filter in a gasoline engine exhaust system.
In this way, the presence of soot in engine exhaust may be detected by oxidizing the soot to generate CO2, and by using downstream CO2 sensors to provide a more direct and more precise estimate of exhaust soot levels, in addition to their use in estimating exhaust CO2 levels. By enabling an accurate, real-time estimate of exhaust soot levels, filter regeneration may be better determined and more accurately initiated. Additionally, the higher sensitivity of the CO2 gas sensors may reduce the “dead-band” effect of resistive sensors and provide higher resolution between signals. This higher resolution may improve the ability to identify a degraded particulate filter. By improving accuracy in filter regeneration and filter diagnostics, the quality of exhaust emissions may be improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.